We proposed a dynamic reassignment scheme of spreading codes for DS-CDMA systems so called RDA (Region Division Assignment) by reconstructing the code tree to mitigate the code blocking problem in order to efficiently provide the services for all supported rates. We compare the performance of the RDA with the conventional OVSF-code assignment scheme (CONV), and the dynamic OVSF-code assignment scheme (DCA). The computer simulation results show that the blocking probability of RDA is lower than that of DCA under the heavy traffic condition. The simulation results also show that the calculation time of DCA is significantly larger than that of RDA. Furthermore, RDA is shown to be able to simultaneously handle much more voice sources than DCA.The co-existence of TDMA and W-CDMA spectrum sharing system (TDMA/W-CDMA overlaid system) with cellular architecture is discussed. In this system, both systems share the same frequency band to improve the spectrum efficiency. BER and spectrum efficiency of the system are calculated for the forward link (down-link) in the presence of AWGN channel. Taking into account the path loss and shadow fading loss, W-CDMA applying interference cancellation (IC) shows a substantial difference in spectrum efficiency than a single multiple access-based system such as TDMA system or W-CDMA system. The interference cancellation can significantly improve BER of the spectrum overlaid system.We investigate the validity of the analytical model of new call channel limiting scheme (NCCL) by means of simulation. NCCL limits new calls to access the total channels with parameter α while handover calls have no restriction to access the channels. We focus on the effects of users' mobility and the parameter α on the blocking probabilities of new and handover good match to the simulation model for low mobility users.